The objective of these studies is to extend our understanding of the pressure and flow characteristics of the pulmonary microcirculation in the newborn under normal and abnormal conditions. The novel technique of micropuncture of the lung will be used to directly measure pressures in the subpleural microcirculation, perivascular interstitium and the alveolar liquid subphase in isolated lungs of newborn lambs. Regional blood flow in the lung will be measured concurrently using radiolabelled microspheres. First, the normal longitudinal distribution of vascular resistance in the pulmonary arteries, veins, and capillaries will be determined in the newborn. Second, the effect of hypoxia on the pulmonary microvascular pressure profile and distribution of blood flow will be studied. This study will determine the differential reactivity of the pulmonary vasculature to hypoxia and identify the site of pulmonary hypoxic vasoconstriction in the newborn. Third, the effect of varying degrees of edema on total and segmental vascular resistance and blood flow to the lung will be studied in the newborn. Fourth, the influence of surface forces on the pulmonary microcirculation in the newborn will be studied. As alveolar surface tension is a major determinant of peri-capillary interstitial liquid pressure, the relationship between alveolar liquid pressure, alveolar surface tension and regional blood flow in the lung will be determined. As newborn infants suffer from a variety of respiratory diseases that are associated with disorders of blood flow to the lung, knowledge derived from these studies may identify means of better treatment and possible prevention of these disorders.